Gamma beam characterization system for ELI-NP: The gamma absorption calorimeter

The ELI-NP Gamma Beam System will deliver an intense gamma beam, in an energy range from 1 to 20 MeV, with unprecedented specifications in terms of brilliance, photon flux and energy bandwidth. A characterization system providing a measurement of the energy spectrum, intensity, space and time profile of the beam is essential for the commissioning and the development of the source, as well as to demonstrate the performance achieved. The technical specifications of the gamma beam prevents to use any traditional gamma spectroscopic detector. A specific system equipped with four basic elements is under development: a sampling calorimeter, for a fast combined measurement of the beam average energy and intensity; a Compton spectrometer, for photon energy spectrum measuring and monitoring; a nuclear resonant scattering spectrometer, for an absolute beam energy calibration and a beam profile imager to be used for alignment and diagnostics purposes. An overview of the ELI-NP gamma characterization system will be given in this work, with a focus on the calorimeter working principle, its expected performances and some preliminary prototype tests.